1. Field of the Invention
The present invention relates to a hammer for use in a hammer mill which is popularly employed in such industrial fields as cement, iron steel, ceramics or thermal power generation.
2. Description of the Prior Arts
Generally in conventional hammer mills, a large number of hammer disks 2a are mounted on a rotary shaft (not illustrated) rotating at high speed (about 400 rpm), and a large number of hammers 1a are oscillatably suspended each from between one hammer disk and another as shown in FIG. 3. The hammers 1a are rotated at high speed along the inner wall of the container according to the rotation of the rotary shaft. Objects to be crushed are fed into the container collide with the front face of each hammer 1a and are crushed by an impulsive force. As a result, the hammer receives a reaction force and rotates around the hammer shaft in the opposite direction so as to prevent itself from breakdown due to excessive impulse.
In this regard, it is a matter of course that the front face directly colliding with the object to be crushed is exposed to such severe conditions that it suffers from strong and considerable impulsive abrasion, so that the life of the front face material is rather short even though a high abrasion resistant material is employed.
To meet the situation, several attempts have been proposed, wherein a hammer is divided into a hammer body and a hammer head so that only the hammer head is replaced when the hammer head is worn out while leaving the hammer body as it is on the shaft.
In a hammer of this type, it is an essential requirement that the replacement of the worn hammer head with a new one should be easily and speedily performed so that interruption in the operation of the hammer mill may be as short as possible. It is also essentially important that the energy applied is not wasted due to looseness between the hammer body and the hammer head.
Referring briefly to the known techniques in the art, Japanese Patent Publication (examinined) No. 42-7234 discloses, as shown in FIG. 4, that replacement of the hammer head is easily performed by providing convex and concave portions slidably engaging with each other in vertical and horizontal directions, and that impulse received by the hammer head 5a at the front end may be absorbed by the turning movement of the hammer body 4b through the engaged portion.
Japanese Utility Model Registration Publication (examined) No. 48-40196 discloses, as shown in FIG. 5, a combination between the hammer body 4c and the hammer head 5c based on substantially the same techincal idea as that noted in the previously mentioned patent publication.
Japanese Patent Publication (examined) No. 44-4568 discloses, as shown in FIG. 6, that the hammer head 5d engages with the hammer body 4d in such a manner as to hold the extending part of the hammer body 4d, then a pin is inserted therethrough and two ends of the pin are welded in such a manner as to secure free oscillation between the hammer body and the hammer head. Thus, impulse forces may be absorbed by the turning movement of the hammer head.
Japanese Patent Publication (examined) No. 45-40632 discloses, as shown in FIG. 7, that to overcome the disadvantage of partial absorption of the driving force due to looseness in the joint between the hammer body 4e and the hammer head 5e of the conventional hammer of the engagement type, the looseness bringing about reduction in driving efficiency, a cutout portion and a tongue piece are provided to be securely coupled with each other so as to receive impulses evenly on the curved surface where the hammer body 4e and the hammer head 5e contact.
It is certain that so long as an integral solid hammer is to be manufactured, the hammer is popularly made of a tough material such as high manganese cast steel. But in the hammer construction which is divided into a hammer body and a hammer head to renew the hammer head alone, there is room for combination of the hammer body and the hammer head each being separately manufactured of different materials.
In manufacturing such a hammer, durability against abrasion may be prolonged several times over the known hammer by manufacturing the hammer head of a material (such as high chromium cast iron) which is more abrasion resistant than that of the hammer body. As is well known, however, high chromium cast iron is not as resitant to impulses and, therefore, there is a possibility of a reduced service life due to breakdown rather than abrasion. As a result, such a hammer comprising a hammer head of high chromium cast iron combined with a hammer body with a similar engagement as the conventional hammer is not suitable for practical use.
For example, backlash or looseness can be seen in the prior art shown in FIGS. 3 and 4 at the engaged part between the hammer head and the mammer body and, moreover, there is a neck part on which load is concentrated in the engaged part. Accordingly, a so-called notch effect takes place in the neck, which means that the mentioned manner of engagement is not suitable for brittle material, either.
In the combination shown in FIG. 5, the hammer head and the hammer body are coupled with each other to be freely oscillatable through a common pin, and impulse is avoided by turning of the two members themselves. However, since every impulse load is imposed directly on the hammer head alone, there still remains concern when using a brittle material.
In the combination shown in FIG. 6, an optimum curved surface at a joint is disclosed to solve the problem of energy loss due to looseness in the coupled part. It is, however, probable that the larger the curved surface area is, the more the possibility of error in the mutually engaged surfaces, and even a small looseness brings about a concentration of an impulse load on a specific part in a complicated way. Such an undesirable result is unavoidable as long as the engaging members are cast products.
In effect, there exists a problem in the state of the art that any of the high abrasion resistant materials (all of them are poor in their resistance to impulse) cannot be adopted in a hammer mill because of the mentioned high possibility of notch, cleavage, cutting or the like.